Recently, analytical techniques that employ small tubular structures (i.e., microcolumns) have gained wide acceptance. For example, capillary electrophoresis (CE) and liquid chromatography (LC), such as high performance liquid chromatography (HPLC), are commonly used techniques for separating analytes, including macromolecules and biomolecules such as proteins, nucleic acids, DNA molecules and fragments, carbohydrates, fatty acids, peptides, and the like. In these techniques, a sample that is suspected of containing analytes is sent through a microcolumn. As the molecules in the sample migrate through the microcolumn, depending on the interaction of the analytes with the other substances (such as packing material) in the microcolumn, the analytes separate from one another.
A very useful technique for detecting analytes passing through a microcolumn (such as the aforementioned) involves directing an incident light to the analytes and detecting the light interaction that results. For example, U.S. Pat. No. 4,675,300 (Zare et al.) describes an electrokinetic process and an apparatus employing coherent radiation-excited fluorescence for detecting analytes. In this system, apparently, laser light is directed at an angle to a translucent section of a capillary to irradiate a sample. The resulting fluorescence light passes through the capillary and is detected by an optical fiber positioned at an angle to the capillary.
U.S. Pat. No. 5,006,210 (Yeung et al.) also describes an analytical system that uses light interaction. Here, analytes in capillary zone electrophoresis are detected by laser-induced indirect fluorescence detection. The laser light is directed at an angle to the capillary and the fluorescence light is also detected by a detector pointing at an angle to the capillary.
U.S. Pat. No. 5,324,401 (Yeung et al.) describes a fluorescence detection system for capillary electrophoresis. This detection system can simultaneously excite fluorescence and substantially simultaneously monitor analyte separation in multiple capillaries. The system has an array of capillaries, each of which has an optical fiber inserted into its outflow end to excite the sample therein. The resulting fluorescence light passes through the capillary walls and is imaged by a CCD camera viewing perpendicularly relative to the axis of the capillary.
In the excitation of a sample and in the detection of light from the light interaction, to increase the signal to noise ratio, it is preferred that more of the excitation light from a light source impinges on the sample and more of the resultant out-going (e.g., fluorescence) light from the sample be collected for detection. In the prior art, as described in the aforementioned patents, the detector is typically directed at an acute or right angle to a detection zone of the capillary in air. In addition, often the excitation light source is also directed at an acute angle to the capillary. The present invention affords a more compact construction and better signal collection than the prior art systems.